Correlation procedures are used for a wide range of tasks in a mobile radio receiver. In a correlation process, the arriving data is compared, in the form of sample values, with a sequence of data items, which are known in the receiver. If the arriving data matches the sequence of known data, a correlation signal is emitted which indicates that the sequence of known data items has been identified in the received signal.
By way of example, the time-slot synchronization in a receiver is carried out on the basis of the correlation of a pilot signal with the pilot sequence which is known in the receiver. In the UMTS Standard, the pilot signal for time-slot synchronization is transmitted via the first common pilot channel pCPICH (primary Common Pilot Channel). The pilot sequence, which is known in the receiver, has a length of 256 chips, and is transmitted at the start of each time slot.
Frame synchronization is likewise carried out by correlation of a transmitted pilot signal with a known pilot sequence. In the UMTS Standard, the pilot sequence for frame synchronization is transmitted via the second common pilot channel sCPICH (secondary Common Pilot Channel). The spreading factor for the second pilot sequence is likewise 256. A third example of the use of correlation procedures for carrying out a mobile radio task relates to the estimation of the delay time for one transmitted signal propagation path. Owing to multipath propagation in mobile radio systems, one and the same transmitted signal is received with a time offset at the receiver, and with different attenuation levels, via different propagation paths. In the course of the equalization process, the time offset between the individual signal components must be measured and must be compensated for (delay estimation). The estimation of the delay time on the propagation path, which is also referred to as delay estimation, is carried out using a product correlation sequence comprising a scrambling code, a channelization code and pilot symbols.
The time-slot and frame synchronizations are used to search for new (mobile radio) cells in the so-called active set (group of currently used cells) and in the so-called monitor set (group of monitored cells which are candidates for the active set). Time-slot and frame synchronizations must therefore be carried out continually even when telephone connection has already been set up. An analogous procedure applies, of course, for delay estimation, which must be continually updated on the basis of changing channel conditions.
Until now, dedicated correlators in the form of matched filter circuits have been provided for each of these correlation tasks in a mobile radio receiver. These matched filter circuits are each designed for only one specific correlation task, and are not interchangeable. They each have an input memory in which the data or sample values to be correlated is or are stored. The sample values are then read from the memory and are fed to a registered chain with intermediate taps (tapped delay line). The taps on the registered chain are connected to multipliers, which multiply the tapped-off sample values by the values of the local correlation sequence. The multiplication results are added and form the correlation signal.
This has the disadvantage that the individual input memories as well as the registered chains cannot be used for two or more correlation tasks, by virtue of their design.